1. Field of the Invention
The present invention relates to a method for preparation of droplets having a uniform size, and more particularly to a method of preparing droplets in a liquid through an orifice without any forced vibration.
2. Description of the Prior Art
Research has heretofore been conducted on methods of preparing droplets wherein a dispersion phase, insoluble or sparingly soluble in an aqueous continuous phase, is injected into the continuous phase through an orifice to form droplets of the dispersion phase having a uniform size, but the droplets obtained have a particle size distribution.
In the method of preparing droplets by injecting an insoluble phase into an aqueous continuous phase in the form of a laminar flow through an orifice, it is known that the diameter of the droplets prepared is dependent on the viscosities, densities and interfacial tension of the two phases.
It is also known that when the velocity of jet of the dispersion phase is controlled to a range slightly lower than the upper limit of laminar flow, the resulting droplets have a particle size distribution of relatively narrow range (Christiansen, R. M. and Hixon, A. N., Ind. Eng. Chem., 49, 1957).
It is further known that the formation of droplets described above closely relates to the phenomenon that when a thread of oil is formed in water by a special method, waves spontaneously occur at the interface and grow into uniformly sized droplets [Taylor, G. I., Proc. Roy. Soc., Ser. A, 146, 501(1934)].
Recently, Kitamura et al. [The Canadian Journal of Chemical Engineering, 60, 723(1982)] report that when a continuous phase is caused to flow coaxially and cocurrently with a jet of dispersion phase from an orifice, with the velocity difference between the two phases reduced at the interface, Taylor waves occur and grow at the interface to form droplets of uniform diameter. Their experiment appears to indicate that when a dispersion phase is injected into a still aqueous continuous phase for the preparation of droplets, the velocity difference between the two phases at the interface presents difficulty in forming droplets of uniform diameter.
On the other hand, Haas [A. I. Ch. E. Journal, 21, 383(1975)] reports that droplets of uniform diameter can be prepared by imparting regular pulsation to a jet of dispersion phase from an orifice to sever the jet at a regular interval.
In the case where such forced vibration is resorted to for preparing droplets of uniform diameter with good stability, it is required to adjust the viscosity of the aqueous phase to not higher than 10 cp and the viscosity of the dispersion phase to about 0.1 to about 10 times the viscosity of the aqueous phase (U.S. Pat. No. 4,444,961).
We have carried out research to obtain droplets of uniform size similarly by forced vibration and found that if the viscosity of the dispersion phase is approximately equal to, or higher than, the viscosity of the aqueous phase, Taylor waves and waves due to forced vibration interfere with each other, making it difficult to produce droplets of uniform size.
On the other hand, when droplets are prepared merely by injecting a dispersion phase into a stationary continuous phase through an orifice, the resulting droplets have a particle size distribution. Uniformly sized droplets can be prepared by the method of Kitamura et al. or of Haas, whereas the method requires too complex an apparatus when to be practiced on an industrial scale.